The present invention relates to monolithic ceramic capacitors mountable on electronic circuit boards. More specifically, the invention concerns ceramic chip capacitors of a variable, free form shape composed of multi layer polycrystalline ceramic and metal composites and the method of making these capacitors.
The basic model of a capacitor, as shown in FIG. 1, is a single plate device 10 consisting of two electrodes 12, 14 insulated from each other by dielectric material 16. FIG. 2 illustrates a basic monolithic ceramic chip capacitor 20. According to known technique, a monolithic ceramic capacitor 20 is manufactured by interleaving electrode layers 22, 24 and dielectric layers 26, compressing all layers, and sintering the layers to form a solid monolithic block.
The principal characteristic of a capacitor is that it is capable of storing electric charge, and this feature is useful in a variety of applications, including, discharge of stored energy, blockage of direct current, coupling of circuit components, by-passing of an AC signal, frequency discrimination and transient voltage and arc suppression. A capacitor""s ability to store electric charge is directly proportional to the capacitance value of the capacitor and the voltage applied to it. Accordingly, to optimize the performance of a capacitor, it is desirable to maximize the capacitor""s capacitance value.
For any given voltage, the capacitance value of a device is directly proportional to the shape and size of the capacitor and the relative permittivity of the medium between the plates. As is evident by the relationship stated above, greater capacitance value can be achieved either by increasing the electrode area or choosing a dielectric material with a high relative permittivity.
In general, capacitors can utilize a variety of dielectrics such as gas (or vacuum), naturally occurring elements (e.g., mica) or prepared materials (e.g., ceramics). Chip capacitors usually utilize ceramic dielectric materials based on the titanates or niobates. The most common dielectric is barium titanate. Numerous attempts have been made to optimize the electrical properties of these dielectric materials by developing new processes and materials. These efforts, however, are costly and time consuming.
An alternative method of optimizing a capacitor""s value is to increase the area of the capacitor. Traditionally, chip capacitors used for surface mounting on electronic circuits have been rectangular in shape. Some attempts have been made to create disk-shaped, crown-shaped or honeycomb-shaped capacitors. These capacitors, however, are costly and difficult to manufacture. Consequently, due to the ease of manufacturability, most capacitors are limited to a rectangular, square or circular shape.
It is an object of the present invention to provide a free form capacitor that will maximize the capacitance value without requiring a change of the materials used in the capacitor.
It is a further object of the present invention to make efficient usage of any available area in which a capacitor will be stored.
It is yet a further object of the present invention to provide a free form capacitor that will be able to conform to the shape of any area and will be able to accommodate any obstacles within that area.
The present invention addresses the problems with the prior art techniques. A free form capacitor is formed by casting tape on a belt casting machine, screen printing the tape with thick film metal ink, and pressing and curing the printed tape stack. The printed stack is then laminated. If it is required by the particular application, internal electrode contact holes can be drilled with a visual recognition drill or route machine. The number of holes and the size of each hole will vary depending on the application of the capacitor. A free form nonsymmetrical capacitor outline is carved using a cutting device. The cutting device could be a routing or drill machine, a razor blade, a laser, a water jet or any similar device capable of forming the outline of the capacitor. The shape of the capacitor is chosen with reference to the item in which the capacitor will be located. The outline of the free form capacitor can comprise of any geometrical shape, including straight lines, angles, convex or concave geometry. Additionally, the various geometrical shapes can exist simultaneously within the same capacitor. After the desired shape is achieved, an organic burn-out is performed.
The surface of the capacitor is cleaned and the corners can be rounded. Ceramic firing or sintering is performed to form the layers into a solid block. If necessary, metallization of holes, sides and surface areas is completed followed by the metallization firing. The application of the metallization is dependent on the efficiency and capability requirements of the particular application.
The free form capacitor of the present invention maximizes the capacitance and other performance parameters that are controlled by space and area. The more area that can be utilized, the higher the capacitance of the improvement over existing product capability. This improvement is much more cost effective than the development of new materials that yield a higher rate of relative permittivity.
The space utilization of the free form capacitor of the present invention results in a higher capacitance value by maximizing the usage of any available area. Additionally, the capacitor can conform to any shape and can form around obstacles within that area.